The contribution of novel pathways of reactive organic carbon to total particulate sulfate and carbon budget

Abstract

Atmospheric chemistry has significant effects on climate and human health due to the formation of atmospheric pollutants. In particular, in-cloud chemistry contributes to the formation of particulate matter (PM) and gas-phase chemistry. Sulfur dioxide (SO2) oxidation leads to sulfate production in cloud water, the second most important constituent of PM. Understanding these oxidation pathways will provide the tools to estimate the sulfate concentrations on a global and regional scale and evaluate the enhancing and quenching mechanism in its formation. Globally, hydrogen peroxide (H2O2) has been considered the dominant oxidant of dissolved SO2, while multifunctional organic hydroperoxides have not been investigated. Organosulfur compounds which compete with sulfate formation have generally been neglected. Research efforts in this field face challenges with measurement techniques for distinguishing organic and inorganic sulfur, especially S(IV) and S(VI) species under dim conditions. This work investigates the aqueous phase kinetic and chemical formation mechanisms of sulfate and organosulfur compounds and presents an improved off-line method that allows separation of S(IV) and S(VI) species in cloud and fog water. An improved ion chromatography (IC) method is presented and compared with aerosol mass spectrometry (AMS) in order to separate sulfate and hydroxymethanesulfonate (HMS), a sulfur(IV) species that can be present in fog and cloud water and contributes to PM composition. The IC method efficiently separates the two compounds, while the AMS achieves efficient separation when HMS is in higher levels than sulfate. The IC method was used to examine the formation of sulfate via the oxidation of dissolved SO2 by multifunctional organic hydroperoxides. The most abundant multifunctional organic hydroperoxides were investigated: the isoprene hydroxyl hydroperoxide (ISOPOOH) and the hydroxymethyl hydroperoxide (HMHP). The two main ISOPOOH isomers, 1,2- and 4,3-ISOPOOH, reacted with dissolved SO2 forming sulfate, with 1,2-ISOPOOH having a comparable rate with H2O2 for pH>5. Under more acidic conditions the reaction did not enhance, and the sulfate yield changed. The chemical mechanism of this reaction is proposed, and the products were identified. HMHP oxidized SO2 rapidly at a higher rate than H2O2 in the cloud relevant pH range. Both pathways contribute significantly to sulfate formation on the regional scale (>50% in isoprene-dominated regions) and when HMHP reaches equilibrium fast it contributes to 18% of the global sulfate production. HMHP and HMS can both be formed in the presence of formaldehyde; thus, the favorable formation conditions of each compound were examined. The rapid oxidation of SO2 by HMHP enables HCHO to act as a catalyst in aqueous atmospheric sulfate formation. Sulfate formation from GEOS-Chem simulations, incorporating the oxidation pathways, was compared with field measurements from the GoAmazon campaign resulting in an improvement in model representation of field measurements. The role of HCHO to HMHP and HMS formation contributes substantially to the global PM budget, with implications for climate and human health.